In recent years, a lighting device for vehicle in which a light emitting diode (LED) is used as a light source, instead of an incandescent lamp (filament lamp), is put to practical use.
In addition, a parallel resistance circuit in which a circuit formed by connecting a resistive element and a thermistor with a positive temperature coefficient in series is connected in parallel to a resistive element which is connected in series to a light emitting diode is proposed.
The thermistor provided in the parallel resistance circuit is operated due to heat which is generated in a resistive element or a protection circuit case when an overvoltage is applied. When an overvoltage is applied, the thermistor enters a non-energized state, regardless of a temperature rise of the light emitting diode, and a current which flows through the thermistor is blocked.
In addition, it is necessary to set a lighting device for vehicle to be used even in a high temperature and high humidity environment in which an environmental temperature is 85° C., and humidity is 85%, for example.
For this reason, when an environmental temperature is high, there is concern that a temperature of a light emitting diode may become excessively high, even when an overvoltage is not applied.
In addition, a temperature compensation circuit which includes a thermistor which is connected in series to a laser diode, and has a negative temperature coefficient, and a resistive element connected in parallel to the thermistor with the negative temperature coefficient is proposed. In this case, when a temperature of the laser diode rises, a light output decreases; however, a voltage applied to the laser diode increases, since a resistance of the thermistor decreases, when a temperature of the thermistor rises. When a voltage applied to the laser diode increases, a light output of the laser diode increases.
As described above, it is necessary to set the lighting device for vehicle to be used even in a high temperature and high humidity environment in which an environmental temperature is 85° C., and humidity is 85%, for example.
For this reason, when a voltage applied to the laser diode is increased in a case of a high environmental temperature, a temperature of the laser diode further rises.
Therefore, a development of a technology in which it is possible to suppress an excessive temperature rise of a light emitting element, even when an environmental temperature is high, is desired.